5,220 research outputs found
Light-regulated adsorption and desorption of Chlamydomonas cells at surfaces
Microbial colonization of surfaces represents the first step towards biofilm
formation, which is a recurring phenomenon in nature with beneficial and
detrimental implications in technological and medical settings. Consequently,
there is a current interest in elucidating the fundamental aspects of the
initial stages of biofilm formation of microorganisms on solid surfaces. While
most of the research is oriented to understand bacterial surface colonization,
such observations at a fundamental level using photosynthetic microalgae are
thus far elusive. Recent single-cell studies showed that the flagellar adhesion
of Chlamydomonas is switched on in blue light and switched off under red light
[Kreis et al., Nature Physics, 2018, 14, 45-49]. Here, we study this
light-switchable surface association of C. reinhardtii on the population level
and measure the kinetics of adsorption and desorption of suspensions of motile
cells on glass surfaces using bright field optical microscopy. We observe that
both processes exhibit a response lag relative to the time at which the blue-
and red-light conditions are set and model this feature using time-delayed
Langmuir-type kinetics. We find that cell adsorption occurs significantly
faster than desorption, which we attribute to the protein-mediated molecular
adhesion mechanism of the cells. Adsorption experiments using phototactically
blind Chlamydomonas mutants demonstrate that phototaxis does not affect the
cell adsorption kinetics. Hence, this method can be used as an assay for
characterizing the dynamics of the surface colonization of microbial species
exhibiting light-regulated surface adhesion.Comment: 10, pages, 6 figure
Macrosystems ecology: Understanding ecological patterns and processes at continental scales
Macrosystems ecology is the study of diverse ecological phenomena at the scale of regions to continents and their interactions with phenomena at other scales. This emerging subdiscipline addresses ecological questions and environmental problems at these broad scales. Here, we describe this new field, show how it relates to modern ecological study, and highlight opportunities that stem from taking a macrosystems perspective. We present a hierarchical framework for investigating macrosystems at any level of ecological organization and in relation to broader and finer scales. Building on well-established theory and concepts from other subdisciplines of ecology, we identify feedbacks, linkages among distant regions, and interactions that cross scales of space and time as the most likely sources of unexpected and novel behaviors in macrosystems. We present three examples that highlight the importance of this multiscaled systems perspective for understanding the ecology of regions to continents
An experimental model of episodic gas release through fracture of fluid confined within a pressurized elastic reservoir
We present new experiments that identify a mechanism for episodic release of gas from a pressurized, deformable reservoir confined by a clay seal, as a result of the transition from bulk deformation to channel growth through the clay. Air is injected into the center of a thin cylindrical cell initially filled with a mixture of bentonite clay and water. For sufficiently dry mixtures, the pressure initially increases with little volume change. On reaching the yield stress of the clayâwater mixture, the lid of the cell then deforms elastically and an airâfilled void forms in the center of the cell as the clay is driven radially outward. With continued supply of air, the pressure continues to increase until reaching the fracture strength of the clay. A fractureâlike channel then forms and migrates to the outer edge of the cell, enabling the air to escape. The pressure then falls, and the clay flows back toward the center of the cell and seals the channel so the cycle can repeat. The phenomena may be relevant at mud volcanoes
Terminal zone glacial sediment transfer at a temperate overdeepened glacier system
Continuity of sediment transfer through glacial systems is essential to maintain subglacial bedrock erosion, yet transfer at temperate glaciers with overdeepened beds, where subglacial fluvial sediment transport should be greatly limited by adverse slopes, remains poorly understood. Complex multiple transfer processes in temperate overdeepened systems has been indicated by the presence of large frontal moraine systems, supraglacial debris of mixed transport origin, thick basal ice sequences, and englacial thrusts and eskers. At Svinafellsjokull, thrusts comprising decimetre-thick debris-rich bands of stratified facies ice of basal origin, with a coarser size distribution and higher clast content than that observed in basal ice layers, contribute substantially to the transfer of subglacial material in the terminal zone. Entrainment and transfer of material occurs by simple shear along the upper surface of bands and by straininduced deformation of stratified and firnified glacier ice below. Thrust material includes rounded and well-rounded clasts that are also striated, indicating that fluvial bedload is deposited as subglacial channels approach the overdeepening and then entrained along thrusts. Substantial transfer also occurs within basal ice, with facies type and debris content dependent on the hydrological connectedness of the adverse slope. A process model of transfer at glaciers with terminal overdeepenings is proposed, in which the geometry of the overdeepening influences spatial patterns of ice deformation, hydrology, and basal ice formation. We conclude that the significance of thrusting in maintaining sediment transfer continuity has likely been overlooked by glacier sediment budgets and glacial landscape evolution studies
One of the closest exoplanet pairs to the 3:2 Mean Motion Resonance: K2-19b \& c
The K2 mission has recently begun to discover new and diverse planetary
systems. In December 2014 Campaign 1 data from the mission was released,
providing high-precision photometry for ~22000 objects over an 80 day timespan.
We searched these data with the aim of detecting further important new objects.
Our search through two separate pipelines led to the independent discovery of
K2-19b \& c, a two-planet system of Neptune sized objects (4.2 and 7.2
), orbiting a K dwarf extremely close to the 3:2 mean motion
resonance. The two planets each show transits, sometimes simultaneously due to
their proximity to resonance and alignment of conjunctions. We obtain further
ground based photometry of the larger planet with the NITES telescope,
demonstrating the presence of large transit timing variations (TTVs), and use
the observed TTVs to place mass constraints on the transiting objects under the
hypothesis that the objects are near but not in resonance. We then
statistically validate the planets through the \texttt{PASTIS} tool,
independently of the TTV analysis.Comment: 18 pages, 10 figures, accepted to A&A, updated to match published
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